Prepulse dependence of X-ray emission from plasmas created by IR femtosecond laser pulses on solids

We report a strong prepulse dependence of the emission of X-rays from plasmas created by 150 fs Ti:sapphire laser pulses on solids. The laser pulses were focused onto plane samples of aluminium and vanadium with a main pulse power density of about 2.5 x 10(16) W cm(-2). By splitting off a portion of the main pulse we were able to generate a prepulse whose delay and intensity were variable relative to the main pulse. With no prepulse, very weak X-ray radiation from Al9+ and Al10+ was observed, whereas a prepulse to main pulse ratio of more than 0.1% and the prepulse preceding the main pulse by more than 2 ns produced up to 50 times stronger X-ray output.